const log = [];

async function a() {
  log.push('a-start');
  await b();
  log.push('a-end');
}

async function b() {
  log.push('b-start');
  await Promise.resolve();
  log.push('b-end');
}

a();
log.push('sync-end');

setTimeout(() => console.log(log.join(',')), 0);